Direct Gas-Fired Industrial Air Heaters are used extensively to provide replacement air to match air that is exhausted or to provide ventilation air in industrial and commercial occupancies. These heaters typically operate around the clock on a year round basis and it is therefore important to minimize the temperature rise of these heaters during mild weather operation so as not to overheat the space. With the airflow held constant as is the case with most make-up air heater applications, the minimum temperature rise relates to the minimum gas flow rate.
In the gas train of a direct gas-fired heater, with the modulating valve deenergized, the gas flow through the modulating valve is adjusted to obtain a minimum flow rate through a bypass circuit provided internal to the modulating valve. It is not unusual to obtain a three (3) to five (5) degree temperature rise as the minimum temperature rise. The basis for determining the minimum temperature rise is that the flame bums over the entire length of burner and that the flame length is long enough to be detected by the flame sense circuit.
Maxitrol Company, Inc. is a company that manufactures the modulating valve and other associated controls that drive the modulating valve electrically from minimum fire to high fire and points inbetween as a function of the discharge temperature of the heater and/or space temperature of the facility being served by the industrial air heater.
In addition, requirements exist from insurance underwriters for this type of equipment, specifically Industrial Risks Insurers, which indicates that ignition and the initial firing rate be limited as defined by the term “Low Fire Start”. General practice of the industry has been to utilize a slow opening (typically a hydraulic operated motor) safety shutoff valve to accomplish a delay in achieving the full firing rate. An alternate means for accomplishing the Low Fire Start had been developed by the manufacturer of the modulating control system, Maxitrol, which involves removing all power from the modulating valve for a short time with a typical delay lasting for ten to thirty seconds.
For burner systems which ignite a pilot light and establish a proper flame signal for the pilot prior to energizing the main burner gas valves, the ignition of the main burner gas is readily accomplished even at the minimum fire condition. In the industry this type of ignition system is referred to as an “intermittent pilot ignition system.” These systems have generally required only one input for supervising or monitoring the presence of flame and that sensor is typically located in close proximity to the pilot flame so as to sense its presence. In some ignition systems, gas flow to the pilot burner would be shut off after adequate time had expired for establishing the main burner flame, thereby having the flame sense circuit actually sense the main burner flame once the pilot flame had extinguished itself. This type of ignition system is referred to as an “interrupted pilot ignition system.”
Direct ignition systems are another means for lighting the main burner gas. In this system, the pilot system is omitted. Ignition of the main burner occurs immediately after the main gas valve is energized. There is a variation of this type of ignition system which may be referred to as a “proven source” type of direct ignition system where current flow to the ignition device is confirmed to be functioning properly prior to opening the main burner gas. All of the above ignition systems have functioned equally as reliably for many years in millions of different heating appliances.
It is generally recognized that a properly designed direct ignition system in a direct gas-fired industrial air heater or make-up air heater application is most difficult or challenging from an engineering standpoint since this type of system is required to ignite the main burner over an extremely wide range of gas flow rates.
To contemplate this aspect of the application challenge in a more detailed manner, one needs to understand that the ignition source, whether it is a high voltage spark or a hot surface ignition device, is generally only present for a few seconds and can be extremely small with respect to the size of burner that it is being utilized on. Gas flow must reach the area of the burner where the ignition source is located with the proper fuel to air ratio to obtain ignition.
During the development of the Harmonized Standard for Direct Gas-Fired Industrial Air Heaters between the United States and Canada, a provision was added that required the main burner flame supervision means for burners over 36 inches in length to be as remote as possible from the ignition source in order to ensure flame propagation has occurred and is maintained over the entire length of burner. To accommodate this requirement in pilot ignition type systems, a second flame detection device can been employed along with the associated controls which switches the pilot sensing system to the main burner flame sense controls after a preset time delay which allows for the flame to propagate across the burner length.
The impact of this provision was found to be more problematic for direct ignition systems with regard to ignition at the minimum fire condition and the time required for that small flame to propagate across the full length of the burner. The flame establishment time period typically only last for only a few seconds after the main gas shut-off valves are energized. The ANSI standard limits the flame establishing time period to a maximum of 15 seconds for direct ignition systems with burners rated over 400,000 Btu/hr and it is understandable why the manufacturer would desire to keep this time as short as possible. Direct fired heaters are not vented and in the case of a delayed or failed ignition, raw gas is dumped into the space being heated. Even though the actual quantity of gas may be small and not pose an unsafe condition for the building or its occupants, the odor from the gas may unnecessarily incite panic to the inhabitants of the building.
Without one of the control methodology provided as the basis for this patent, the minimum gas flow adjustment would have to be significantly increased or other more expensive gas flow controls systems employed for direct ignition type systems to ensure that the flame would propagate across the burner within the flame establishment time period. Longer burners would require a higher minimum fire adjustment to account for the distance that the flame has to travel. The downside of increasing the minimum gas flow rate is that the minimum temperature is increased which then results in overheating of the conditioned space during mild weather conditions.
An alternate control approach mentioned above for gas flow control would involve providing a separate gas piping system which would be energized for each for each ignition attempt and provide the flow of gas necessary to achieve the flame travel speed to complete the flame sense circuit before the flame establishing time period expires. It was noted that this solution was significantly more expensive than the other control methodologies presented within the scope of this patent coverage. This is because the separate gas piping system would require a gas valve with redundant valve seating and a regulator and/or flow regulating cock to simulate a variable orifice, either or both provided as a means to adjust the gas flow precisely to obtain the desired effect. In addition, a time delay relay would be necessary to energize the primary gas controls for the heater after the flame had been properly established and de-energize the low fire start controls. In this type of arrangement, a low fire start setting can be employed without sacrificing the lowest possible minimum fire setting, thus the minimum temperature rise aspect of the make-up air heater is maintained.
The current invention has been designed to provide a less expensive solution for direct ignition control system while maintaining the minimum firing rate at the lowest possible and achieving consistent ignition performance at a pre selected “low fire start” setpoint.